The present invention concerns a hot chamber die-casting apparatus which works under pressure and is used to cast articles of pure aluminum or its alloys as well as pure zinc, magnesium, copper and their alloys and all other alloys which strongly corrode ferrous materials. The apparatus permits the production of articles with an excellent surface finish without inclusions of oxides or anything else and without any contamination from the apparatus itself. In thin sections the articles produced are of uniform quality, improved ductility, and unweakened by iron dissolution.
In casting under pressure, two processes can be distinguished: cold chamber die casting and hot chamber die casting. The former is characterized by the fact that the molten metal is ladled into an unheated injection cylinder before each filling of the die. The second process is characterized by the fact that the injection cylinder is at the same temperature as the molten metal. The main disadvantages of the cold chamber process are:
when the molten metal is transported from the holding furnace to the chamber, a certain amount of oxide is simultaneously transferred PA1 it is difficult to determine the exact quantity of molten metal ladled, PA1 there is a variation of the oxide content of the molten metal which influences the quality of the parts, PA1 further oxidation of the molten metal occurs during the filling of the injection cylinder, PA1 metal contamination by the lubricant of the injection cylinder occurs, PA1 there is no regular temperature evolution during casting. PA1 more regularity in the metal temperature at the die entrance, PA1 the injection cylinder is filled without contact between the air and the metal being injected, PA1 a uniform composition of liquid results. PA1 higher productivity results due to better utilization of the apparatus and a lower scrap rate regarding the casting and secondary operations. PA1 the need for hand-ladling or auto-ladling is eliminated, PA1 there is a greater potential for automation, PA1 reproducible, predictable and controllable casting cycle results. PA1 there are multiple possibilities of positioning the die, PA1 less internal pressure results, PA1 there is less thermal variation as a result of faster and more uniform cycles, PA1 there is less shrinkage of the casted part in the die, PA1 the apparatus has longer life. PA1 piston and cylinder corrosion, abrasion, and mechanical constraints such as loosening, poor fitting, alignment and others. PA1 cylinder-end and gooseneck corrosion and erosion, PA1 nozzle corrosion, erosion, oxidation in air, mechanical and thermal shocks, PA1 crucible corrosion and oxidation in air.
The advantages of the hot chamber process are:
Concerning Production
Concerning Die Peformance
While there has been success in the hot chamber die casting of lead, tin and zinc alloys by way of a piston pump injection cylinder, the application of this technique has proved to be unsuccessful as far as aluminum and other corrosive metal alloys are concerned because ferrous material are quickly attacked by these metals when molten, thus limiting the life of the pump.
A gooseneck air machine has been used for aluminum alloys in order to obtain the advantages of the hot chamber process. However, the castings thus produced are not satisfactory in porosity and show increased iron content, the latter increasing the brittleness of the castings. At the present time, the casting of aluminum under pressure is essentially done with the cold chamber process, which is limited by the problems which arise from its unadaptability to automatization. The hot chamber process with the piston pump is a simple process which could make automatization possible (as in the case of zinc die casting) and thereby enable more economic production with better uniform quality.
Nevertheless, molten aluminum alloys are so corrosive that on the market at the moment, there is not a single properly adapted hot chamber die pump for casting these metals under pressure depite the encouraging results already obtained.
Indeed, Union Carbide Corporation has filed two patents (U.S. Pat. No. 3,319,702 and U.S. Pat. No. 3,586,095) concerning piston pumps, the elements of which (the piston and the cylinder), being very exposed to erosion and corrosion, are made of sintered TiB.sub.2 and ZrB.sub.2, and can withstand 50,000 cycles. Such pumps produce an increase of the iron content in the alloy which is less than 0.05% after 150,000 cycles (C. F. Fulgenzi, Trans Soc of Die Casting Eng 1968 paper 46). However, prolonged tests of these pumps on production machines (P. W. Marshall, Foundry Trade Journal, Nov. 26, 1970, 797) showed that their lifetime was irregular (from 600 to 57,000 cycles). The main reasons for the early degradation of the piston and cylinder are the poor resistance of TiB.sub.2 and TrB.sub.2 to thermal shock and the extremely large thermal expansion differences between cast iron and these materials which, in the long run, free-play and out of roundness of the parts. Again, the increase in the iron content of the alloy is very small (0.2%). This increase is nevertheless too large if alloys which contain very little iron are to be cast.
A Japanese patent (J P 74 074 523) describes an invention concerning a pump to be used only in aluminum casting which has been tested with molten aluminum. It can endure 120,000 cycles under a pressure of 150 kg/cm.sup.2, with a piston and cylinder made of hot-pressed TiC (75%)+Si.sub.3 N.sub.4 (25%) and 160,000 cycles under a pressure of 120 kg/cm.sup.2 with a piston and cylinder made of hot-pressed TiC (52%), Si.sub.3 N.sub.4 (25%), TiB.sub.2 (20%), and CrB.sub.2 (3%).
A German patent (2 320 887) discloses the use of pump parts made of AlN alloyed with 0.1 to 10% in weight of Y.sub.2 O.sub.3, La.sub.2 O.sub.3, Sc.sub.2 O.sub.3, Ce.sub.2 O.sub.3, Al.sub.2 O.sub.3 or metallic silicates. Pistons and cylinders made of these hot pressed substances have a lifetime of 100,000 to 130,000 pumping cycles of molten aluminum alloy.
A Japanses patent registered in Switzerland under the number CH 586 581 describes an injection pump characterized by a piston and cylinder or a coating of the latter which are manufactured from a sintered body consisting of a mixture of carbides and borides such as boron carbide 10 to 90% in weight, preferably 30-70%, titanium boride 5-60%, zirconium boride 5-60%, boron nitride 0.5-30%, and possibly 0 to 5% tantalum, molybdenum, tungsten borides, zirconium, silicon, tantalum, vanadium, chromium, tungsten, molybdenum carbides, aluminum, silicon, titanium, zirconium nitrides, aluminum and beryllium oxides.
The porosity of the sintered body is lower than 5%, its resistance to flexion above 400 kg/cm.sup.2 and its hardness above 1400 kg/mm.sup.2.
Tests with piston and cylinders made of such materials have been carried out. In certain cases, the main chamber body of cast iron, covered with graphite to protect it, becomes corroded by the fused metal (Al, 1.5-3.5 Cu, 10.5-12 Si, 0.3 Mg, 1 Zn, 0.9 Fe, . . . ) after 110,000 to 160,000 injection cycles under a pressure of 150-250 kg/cm.sup.2, although no sign of corrosion on the cylinder or the piston has been detected. However, it seems that this pump can only be used to cast zinc and magnesium alloys.
Thus it seems that there are certain materials adapted to the construction of the piston and cylinder even though they are not entirely satisfactory. However, the life of the pump and the quality of the cast pieces remain limited due to the corrosion and wearing out of the parts of the pump other than the piston and cylinder. The problems that must be solved in order to realize a pumping apparatus on a piston pump correspond to the four parts of this pump and may be stated as follows:
The materials to be used to realize such a pump must be resistant to corrosion by molten aluminum, to abrasion and erosion, be quite hard and process expansion coefficients similar to those of ferrous materials. Also, for certain uses already stated, they must be able to resist oxidation and thermal and mechanical shocks. Finally, it is desirable to be able to use materials which are not wetted by molten metal and which are as dense as possible.
Table I shows several known properties of the materials which are the most resistant to molten aluminum corrosion, but other properties must also be taken into account as far as the realization of a pump is concerned.